1. Field of the Invention
This invention relates to a method and system for correcting the preprogrammed path of a robot manipulator to compensate for variability in the positioning of a workpiece.
2. Description of Related Art
A plethora of different types of robot manipulators is used in the industrial setting for performing a variety of tasks such as spray painting, spot welding, sealing, parts picking, and other operations. A common setting involves the use of a robot manipulator to perform the same operation upon a succession of workpieces. This requires that each workpiece be received into the workstation of the robot and be fixtured in place during the robot's operation upon the workpiece.
Certain operations performed by industrial robots, such as spray painting, do not require fine accuracy in terms of the positioning of the workpiece vis-a-vis the robot. Other types of operations such as sealant application, however, require that considerable precision be maintained in terms of the path swept by the robot in relation to the workpiece. Such precision may be obtained in two basic manners. First, the workpiece may be fixtured in place by means of a rigid holding mechanism. Although such mechanisms will generally produce acceptable results in terms of the placement of adhesives or other materials upon a workpiece, such equipment usually has an undesirable characteristic inasmuch as the time required for fixturing the workpiece adds appreciably to the total cycle time required for the operation.
An alternative to rigid fixturing of a workpiece involves placement of the workpiece in the approximate position required for the given operation followed by correction of the robot path to compensate for deviations from the idealized location of the workpiece. This is generally accomplished in a two-step process with the first step involving sensing of the precise location of the workpiece followed by correction of the robot path based upon the results of the sensing step. U.S. Pat. Nos. 4,017,721; 4,146,924; 4,380,696; 4,402,053; and 4,575,304 all disclose various details of systems which may be used for the purpose of sensing the position of a workpiece. These systems do not include a path correction method of the type disclosed herein. U.S. Pat. Nos. 4,432,063 and 4,433,382 disclose robot manipulators and control systems providing for movement of a robot manipulator on a piecewise digression from a preprogrammed path, followed by resumption of the program path. These systems do not include a system for correcting a robot path in the wholesale fashion taught by this specification. Finally, U.S. Pat. Nos. 4,639,878 and 4,707,647 disclose methods and systems for sensing the placement of a workpiece and for correcting the path of a robot manipulator based upon the location of the workpiece. The systems disclosed in the '878 and '647 patents require the solution of simultaneous equations by numerical techniques in order to obtain location data defining the position and attitude of the workpiece with respect to a fixed coordinate system. Such a method is both costly and time consuming.
It is an object of the present invention to provide a method and system for correcting a preprogrammed robot manipulator path to correspond with the various locations of a succession of workpieces without the need for solving excessive numbers of equations each time a workpiece is docked into the manipulator's work station.
It is an advantage of the present invention that a method and system according to this invention will allow speedy correction of a robot path with economical usage of computer resources.
It is a further advantage of the present invention that a system operating according to this invention is able to use a small number of measurement points, for example, one or two points, depending upon the type of operation being performed, the architecture of the workpiece and the type of workpiece fixturing employed. Two points will provide a sufficient amount of data for the sealing or welding of a substantially straight seam located on a workpiece. In contrast, the systems disclosed in the '878 and '647 patents always require data from at least three measurement points.
Other objects, features and advantages of the present invention will become apparent to those reading this specification.